This invention relates to line printers, and more particularly to the control of electrical energization of the thermal head of a printer.
The arrangement of a conventional printer of this type will be described with reference to FIGS. 7 and 8.
FIG. 7 is a block diagram showing the arrangement of the conventional printer, and FIG. 8 is a circuit diagram showing the thermal head of the conventional printer.
The conventional printer, as shown in FIG. 7, comprises: a CPU 1; a ROM 2 connected to the CPU 1 through a bus; a RAM 3 connected to the bus; a parallel data input interface 4; a counter connected to the bus and the parallel data input interface 4; an input/output port 6 connected to the bus and the parallel data input interface 4; a line buffer 7 connected to the parallel data input interface 4; a P/S (parallel to serial conversion) circuit 8 connected to the line buffer 7; a latch circuit 9 connected to the P/S circuit 8; a latch circuit 10 connected to the latch circuit 9; a P/S circuit 11 connected to the line buffer 7; a latch circuit 12 connected to the P/S circuit 11; a latch circuit 13 connected to the latch circuit 12; a P/S circuit 14 connected to the line buffer 7; a latch circuit 15 connected to the P/S circuit 14; a latch circuit 16 connected to the latch circuit 15; a ROM 17 connected to those P/S circuits 8, 11 and 14 and latch circuits 9, 10, 12, 13, and 16; a switch circuit 18 connected to the ROM 17; an AND circuit 19 connected to the switch circuit 18 and the latch circuit 15; a thermal head 20 connected to the AND circuit 19; and a head temperature detecting circuit 21 whose input and output are connected to the thermal head 20 and the input/output port 6, respectively.
The thermal head 20, as shown in FIG. 8, comprises: 2048 heat generating resistance elements R1 through R2048; 32 shift registers LSINO.0 through LSINO.31; power terminals: a CLOCK terminal; and an HLTH terminal.
Now, the operation of the conventional printer thus constructed will be described.
Printing data are applied through the parallel data input interface 4 to the line buffer 7. The data of an aimed dot in a line to be printed is applied, as an output C3 of the latch circuit 15, to the AND circuit 19. The data of the dot before the aimed dot is applied as an output C2 to the ROM 17, and the data of the dot after the aimed dot is applied as an output C1 to the ROM 17.
The data of an aimed dot in the line before the line to be printed is applied, as an output B3, to the ROM 17. The data of the dot before the aimed dot is applied, as an output B2, to the ROM 17, and the data of the dot after the aimed dot is applied, as an output B1, to the ROM 17.
The data of an aimed dot in the line which is located two lines before the line to be printed is applied, as an output A3, to the ROM 17. The data of the dot before the aimed dot is applied, as an output A2, to the ROM 17, and the data of the dot after the aimed dot is applied, as an output A1, to the ROM 17.
The electrical energization time control of the thermal head 20 will be described with reference to FIG. 9.
FIG. 9 is a flow chart for a description of the electrical energization time control of the conventional printer.
In Step S10, the CPU 1 detects the temperature of the thermal head 20 with the aid of the head temperature detecting circuit 21. The temperature detection data is stored in the RAM 3.
In Step S11, the printing interval of each line; that is, a recording period is obtained, and stored in the RAM 3.
In Step S12, the real record dot number of a line to be printed is obtained.
In Step S13, a degree of thermal effect on the printing line is obtained according to the above-described recording period and real recording dot number;
In Step S14, the above-described degree of thermal effect is obtained for each of the first to current (present) lines, and the degrees of thermal effect thus obtained are stored.
In Step S15, a correcting value is obtained according to the degrees of thermal effect thus stored and the head temperature.
In Step S16, an electrical energization time is obtained according to the head temperature and the recording period, and the above-described correcting value is used to obtain a fundamental energization time T1. In addition, adjusting energization times T2, T3 and T4 are obtained according to the outputs A1, A2, A3, B1, B2, B3, C1 and C2 of the latch circuits, when necessary.
In Step S17, the thermal head 20 is energized according to the fundamental energization time T1 and the adjusting energization times T2, T3 and T4.
In Step S18, the above-described operations of Steps S11 through S17 are carried out repeatedly until the printing operation is accomplished.
The electrical energization time control will be described with reference to FIGS. 10 and 11 in more detail.
FIG. 10 is an explanatory diagram showing the data patterns of dots surrounding an aimed dot, and FIG. 11 is a timing chart indicating printing timing with the data patterns shown in FIG. 10.
The parts (a) through (h) of FIG. 10 shows eight typical states of dots surrounding an aimed dot P in a line l to be printed. In FIG. 10, reference characters l-1 designates the line before the line l to be printed; and l-2 designates the line located two lines before the line l. Furthermore, in FIG. 10, hatched dots are to be printed black.
In the case of the part (a) of FIG. 10, the dots on both sides of the aimed dot P, and the dots on the lines l-1 and l-2 are not printed. In this case, the heat generating resistance element for the aimed dot P is not affected by the heat of the other dots at all, and therefore the energization time is the sum of the fundamental energization time T1 and the adjusting energization times T2, T3 and T4 as shown in the part (j) of FIG. 11.
In the case of the part (b) of FIG. 10, the dot on the left side of the aimed dot P is printed. In this case, the heat generating resistance element for the dot affects the one for the aimed dot P, and therefore in the total energization time, the adjusting energization time T2 is off as shown in the part (i) of FIG. 11.
In the case of the part (c) of FIG. 10, the heat generating resistance element for the aimed dot P has printed black dots on the preceding line l-1. In this case, in the total energization time, the adjusting energization time T3 is off as shown in the part (h) of FIG. 11.
As can be estimated from the above description, in the cases of the parts (d), (e), (f), (g) and (h) of FIG. 10, the energization times are indicated in the parts (g), (f), (e), (d) and (c) of FIG. 11, respectively. The parts (a) and (b) of FIG. 11 show printing data, and latch signals, respectively.
In the above-described case, reference is made to dots other than end dots of every data input of the thermal head 20 with eight reference dots of an aimed dot taken into account.
Now, a reference method in which reference is made to end dots of each data input of the thermal head will be described with reference to FIGS. 12 through 16.
FIG. 12 is an explanatory diagram showing separation of the heat generating resistance elements of the thermal head 20 shown in FIG. 8. FIG. 13 and FIG. 14 are explanatory diagrams showing the arrangement of memory in the line buffer 7. FIG. 15 is a block diagram showing a conventional reference circuit. FIG. 16 is an explanatory diagram showing reference timing in the prior art.
The 2048 heat generating resistance elements R1 through R2048 are connected to the serial in 64-bit shift registers LSINO0 through LSINO31, and each of the data inputs HDI1 through HDI8 is handled by four shift registers; that is, each data input has 256 bits.
For the purpose of high speed printing, two data inputs form an electrical energization block; that is, 512 bits can be energized at the same time.
Thus, as shown in FIG. 8, the 2048 heat generating resistance elements are divided into four energization blocks HSB1, HSB2, HSB3 and HSB4.
The thermal head 20 is designed as shown in FIGS. 8 and 12. Therefore, the arrangement of memory of the line buffer 7 is allocated to data inputs of the thermal head 20, and in the data inputs, the data corresponding to the heat generating resistance elements R1 through R2048 are allocated as shown in FIG. 14, being transferred, 8 bits by 8 bits, from the parallel data input interface 4.
As shown in FIG. 15, the conventional reference circuit has two blocks equation in arrangement so that two data inputs can be transferred simultaneously. One of the two blocks is for the data inputs HDI1, HDI3, HDI5 and HDI7 of the thermal head 20, and the other is for the data inputs HDI2, HDI4, HDI6 and HDI8.
The conventional reference method for an end dot of each data input is such that, for instance in the case where the aimed dot is R256(l) in FIG. 16, reference is made to only five of the eight reference dots. Similarly, in the conventional reference method, for an end dot of each data input such as the aimed dot R257(l) or R512(l) reference is made to five dots, for energization control.
As was described above, in the conventional printer, for an end dot of each data input, reference is made to only five of the eight reference dots to perform energization control. Therefore, the resultant print includes a printing defect such as a vertical stripe for every end dot.